In recent years, with progress of reduction in size and weight and increase of functions and processing speed of electronic apparatus, the reduction in size, increase of integration and dissemination of surface mounting technology of various electronic parts have advanced and a demand for higher integration and higher reliability of printed circuit boards has increased.
For producing printed circuit boards, it has been proposed that a resist composition is printed on a substrate having copper surface to form a patterned resist layer as etching protective layer, and then etching is conducted to obtain a patterned copper circuit board. It has also been proposed that, instead of printing the resist composition, the resist composition is coated on the substrate having copper surface and then exposed to light and developed to form a patterned resist layer. The coating can be conducted by electrocoating.
Electrocoating is a method wherein a substrate is dipped in an electrodeposition bath which contains a water soluble or water dispersible photosensitive resin composition, and an electric current is applied as giving an opposite charge to a substrate to be electrocoated to form the photosensitive resin composition layer on the substrate. Electrodeposition coating has such advantages that it is possible to form a film with uniform thickness on the copper surface of the substrate, to enable easy control of film thickness by adjusting applied voltage and applied period of time, to obtain good follow-up of the film to the indentation of and injury of the copper surface which makes special pretreatment of the copper surface of the substrate unnecessary, etc. Even when small diameter through-holes or via-holes exist in the substrate, the electrocoating composition infiltrates into such holes to form the photosensitive resin layer which becomes a sufficient protective film against the etching liquid. Besides, automation of the process is easy and through processing line from plating up to the step before the light exposure can be established.
However, in the electrodeposition coating, the film immediately after electrodeposition is wetted by residual moisture, solvent and undeposited coating liquid. Consequently, it is general to rinse the film with ion exchanged water to remove the undeposited coating liquid and subsequently to heat-dry to remove the moisture and water.
However, if water rinsing or dewatering after water rinsing is insufficient, water drop marks or unremoved coating liquid marks remain on the electrodeposited film. Especially in the case of positive type electrodeposition resist film, the marks remain without being removed by developing or etching even when the part in question has been exposed to light, and they become the cause of shorting between wirings. Also, around the circular periphery of water drop marks, pinholes often generate.
These troubles cause fatal damage to the manufacture of highly integrated printed circuit boards since the conductor pattern of the board has become finer and finer recently. Thus, in the actual industrial producing line, number of water rinsing steps increases, but it is not sufficient in view of a space for equipments. In order to prevent the water drop marks, air-blowing after washing has been proposed but depending on an angle and position of an air-blower, a size or shape of the substrate or due to spreading of water drops caused by a complicated shape of jigs fixing the substrate during transportation, water drop marks may easily remain on the film. It is therefore highly difficult to completely prevent generation of the water drop marks.
In order to avoid the above mentioned water drop marks, Japanese Kokai Publication 2(1990)-39050 discloses a method wherein a conventional electrocoating composition is electrocoated on a substrate, and then a different electrocoating composition composed of watersoluble or waterdispersible resin as a main polymer is further electrocoated. This method is effective in preventing generation of water drop marks, but since it necessitates the additional installation of electrodeposition equipment, installation cost of the system is high and incorporation of the equipment into the existing electrodeposition line accompanies the problem of modification of installation space and operation program.